The simple fact that, when looking back on what you did, it is very clear what you should have or could have done instead and made things work out infinitely better.

Unlike the future, the recent past is literally and figuratively an open book to us - we see it with the clarity of having seen its consequences as well, and 20/20 is simply another way of expressing that clarity. It is a measure of what is considered normal eyesight. If you have 20/40 vision, you have to stand 20 feet from a sign to read it, while a normal person could stand at 40 feet and read it.
In some writings of fantasy, there are beings that see the future with clarity, but have no memories of the past - once an event has happened, they have no knowledge of it (see foregoblin). Foresight is 20/20.

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